A conventional communication system achieve communication between movable bodies (slave nodes) and a control device (master node), which controls movement of the movable bodies, via a power supply line for supplying power to the movable bodies in a non-contact manner.
Specifically, a communication signal is superimposed on an alternating-current power supply signal, and the communication signal is input and output via communication antenna coils disposed in the movable bodies. Signals transmitted from respective movable bodies and signals received by respective movable bodies have different frequencies. Thus, communication between the movable bodies is achieved via a relay process by the control device that receives a signal and transmits the signal with changing frequency (see, for example, JP-A-H10-84303 corresponding to U.S. Pat. No. 6,005,475A).
In the above-described communication system, the slave nodes cannot directly communicate with each other. Thus, the communication system has a low responsivity and is unsuitable to event directional communication. In addition, the master node needs to manage address information and the like of all the slave nodes for the relay process. When a slave node is additionally coupled, the master node needs to be reconfigured. Therefore, a system configuration cannot be changed easily.
When a multi-master method in which nodes can directly communicate with each other without via a master node is applied to a communication system in which nodes are coupled to a transmission path in a non-contact manner, the following issues are caused.
In the multi-master method, a transmission signal and a reception signal on the transmission path needs to have the same frequency, and signals transmitted from the respective nodes may collide on the transmission path. Thus, the collision needs to be detected.
As a method of detecting the collision, a carrier sense multiple access with collision detection (CSMA/CD) method and a carrier sense multiple access with collision avoidance (CSMA/CA) method are known. In the CSMA/CD method and the CSMA/CA method, each node directly detects a level of a signal on the transmission path and detects a collision based on determination of whether a signal transmitted from the node corresponds to the signal on the transmission path.
However, the node coupled to the transmission path in the non-contact manner cannot directly detect the level of the signal on the transmission path. Thus, the conventional method cannot be applied simply.